The volume change between an undeformed and deformed configuration induces a pressure change described by an equation of state. Gases and liquids typically exhibit different compressibility, and are thus modeled differently. Common compressibility models are built-in into the Enclosed Cavity node, and are described below.

For adiabatic expansion or compression of an ideal gas, the deformed volume of the cavity V is related to its initial volume Vref by the equation of state

Here, pref is the reference fluid pressure inside the undeformed cavity, p is the absolute pressure after deformation, and γ is the ratio of specific heats. Since p = pref + prel, this relation is equivalently written as

where prel is the relative pressure acting on all cavity walls.

For isothermal expansion or compression of an ideal gas, the pressure and volume of the deformed cavity are related to the reference pressure and volume by Boyle’s law:

For a nearly incompressible fluid, the fluid compressibility χf is considered much smaller than the compressibility of the surrounding solid, for instance, a thousand times smaller than a representative bulk modulus.